A conventional motor starter device for a refrigerator or air conditioner compressor is shown in FIG. 8 along with motor 100 having a start winding S, a main winding M and a common terminal C. A positive temperature coefficient of resistivity thermistor (PTC) 110 is connected in series to start winding S and an overload protection device 120 is connected to terminal C. An operating capacitor CR is connected between start winding S and main winding M for increasing the efficiency of the motor.
During startup of the motor, sufficient current flows through start winding S since the resistance of the PTC thermistor is small at normal temperatures. After startup, PTC thermistor 110 generates heat on its own because of the current that flows through it which results in a sudden elevation of the thermistor resistance, thereby assuming a state of high resistance with an electric current of several tens of milli-amperes. Upon an overload or a restrained operation of motor 100, the overload protection device 120 opens the circuit as a result of the excessive current and/or winding temperature.
In the case of such a motor starter device, PTC thermistor 110 is maintained at high temperature and high resistance even during the normal operation of the motor, thereby limiting current to start winding S. As a result of this, several watts of power are wasted due to this holding current through PTC thermistor 110. Various attempts have been made to solve this problem. U.S. Pat. No. 5,898,289 shows, as seen in FIG. 9, a startup PTC thermistor 230 and a triac 240 connected in series with start winding 220 of motor 200 having a main winding 210 and a start winding 220. PTC thermistor 250 placed in parallel with startup PTC thermistor 230, is connected to gate terminal G of triac 240 for control of the triac. PTC thermistor 250 turns the triac 230 off after the motor starts for the regular operation of the motor thereby reducing power consumption of startup PTC thermistor 230.
U.S. Pat. No. 5,451,853 discloses a starter device for reducing power consumption by connecting an RC time constant circuit at the gate of a triac that has been connected in series to the startup PTC thermistor allowing the triac to conduct until the capacitor has been fully charged.
Japanese Toku Kai Hei 5-328767 further describes a trigger circuit connected to the gate of a triac that has been directly connected to the startup PTC thermistor, with the gate voltage lowered by the trigger circuit after the passage of a prescribed period of time following the startup of the motor, with the triac being turned off. The trigger circuit makes it possible, for example, to set the time by means of the RC time constant circuit or set the elapsed time using the time of the motor startup.
However, the conventional motor starter devices described above have the following limitations. With respect to the starter device shown in '289 patent, a trigger signal is impressed on triac 240 through PTC thermistor 250 for triac control when the motor is started up, triac 40 being turned on, with a result that the current flows to startup PTC thermistor 230. After a prescribed period of time following the startup, the resistance of PTC thermistor 230 increases due to the heat it generates and, in the case of the PTC thermistor 250 for triac control, its resistance value rises and the current that is impressed on the gate terminal of the triac 240 is lowered, with a consequence that the triac 240 turns off.
However, because triac control PTC thermistor 250 is connected in parallel with the startup PTC thermistor 230, the current flows through triac control PTC thermistor 250 even after triac 240 has been turned off, making it impossible to control the power consumption for this portion of the circuit.
In addition, there are cases where the triac control PTC thermistor 250 can be affected by outside temperature, making it impossible to carry out stable operation. If the ambient temperature happens to be high, the temperature of the triac control PTC thermistor also rises, with a result that triac 240 is turned off earlier than anticipated. When a re-startup is effected after an overload operation or a restrained operation, for instance, the surroundings of the motor may not be sufficiently cooled and this is especially the case during the summer. Because of this, it is not possible to satisfactorily start the motor, making it necessary to repeat the startup procedure several times.
The time constant circuit or the trigger circuit for control of the state of conductivity of the triac is not directly responsive to the action of the motor and controlling the triac as shown in the '853 patent or the '767 patent references, with a consequence that the operation of the triac becomes unstable. To avoid this, it becomes necessary to set the time for the triac to turn off longer which results in a problem of not being able to start the motor quickly when re-starting the motor.